It's probably due to a combination of problems...
We've been talking about dirty LPG recently, contains dissolved oil ('heavy ends') which comes out of the LPG during evaporation inside the reducer, flows along with gas vapour to injectors, when cold turns into a sticky tar like substance that can prevent or slow the speed of injectors opening https://lpgforum.co.uk/viewtopic.php?t=19640
The reducer could be another problem. Maybe also due to heavy ends, maybe not... I've known quite a few Antarctic reducers have the problem of slightly sticking open gas inlet which sees pressure rise to 3 bar when there is no gas being drawn off (when the gas inlet port should be closed). The gas inlet port could be sticking open due to heavy ends. Or if you haven't monitored the gas pressure reading before the gas injectors start firing for a while, maybe it's been 3 bar before the injectors for a long time but still switched to gas OK until recently in which case we couldn't attribute all your recent problems just to the high gas pressure (although it would never help).
Don't know what ECU you're using but another problem can be that if you have set a 1.3 bar reference pressure we'd expect the ECU's default (none adjustable on some ECUs) pressure correction to compensate Ginj (gas injector pulse length) by around negative 50% if the pressure reading is 3bar, the default ECU settings are based on physics but don't take into account that the higher pressure will also have the effect of slowing the speed of injectors opening, so at such a high pressure (that will make a big difference to speed of injectors opening) the negative 50% is vastly excessive and has the effect of leaning the mixture due to too short Ginj. If the over-pressure situation is momentary (say the pressure decreases rapidly as soon as injectors start opening) it can be better to dial in a positive (obviously as opposed to negative but I mention it to highlight the point) pressure compensation for pressure readings above the usual range that will be seen during normal engine operation to prevent the over-pressure from leaning the mixture and also to give the injectors a longer pulse to give them better chance of opening and bleeding some of that pressure when they are first pulsed during changeover.. If possible I'd go with the standard pressure corrections up to 1.8 bar, then a zero correction to 2bar, then a positive correction up to around +15% at 3bar. Heh, my +15% correction for +3bar will be around a 65% longer pulse than the ECU would give with standard pressure correction. At cold changeover there'll also be some negative Ginj correction because besides pressure cold gas is denser than hot gas. That said, some ECU's also have a default minimum injector pulse length for the type of injectors you've selected in software (and some others allow a minimum pulse length to be defined by the installer). What ECU are you using?
Could be one I converted, I don't remember whether I did yours or not lol.